This invention relates to a power supply for an intrinsically safe loads. More particularly, this invention relates to circuitry that limits the current supplied to the load to meet intrinsically safe standards.
Electronic devices are often used in hazardous environments containing volatile material. It is often a problem that a spark or heat from the electronic devices can cause the volatile material to ignite. Therefore, makers of electronic devices for use in these hazardous environments must provide some protection to ensure that the electronic devices do not ignite the volatile material.
One such form of protection is to make a circuit intrinsically safe. Intrinsically safe standards are set by regulating authorities such as the UL in the United States, CENELEC in Europe, CSA in Canada, and TIIS in Japan. In order to be intrinsically safe the current, power, and voltage through the circuit are limited to levels that prevent ignition of the volatile material from a spark or heat generated by the circuit.
It is a problem to deliver electricity to an intrinsically safe device. The power, voltage and current of the electricity are limited to the levels insufficient to ignite the volatile material. Therefore, components are needed in the power supply to limit the power, voltage, and current delivered to the intrinsically safe device.
In a conventional power supply, voltage is limited by connecting one or more zener diodes between supply line connecting a power supply to output terminals. The zener diodes limit voltage to V. Current limiting is provided by connecting a resistor having a resistance of R in series with a high potential output terminal. The resistor limits current to V/R. Power is controlled by the limiting of current and voltage.
The limiting components, i.e. the diodes and resistors, must be protected to prevent the components from exceeding published rating in the event of a fault. A fuse is typically added to the circuit to limit the amount of current that can be delivered to the components. A fuse is chosen that has a rating that ensures power dissipation ratings are not exceeded.
Although not required for intrinsical safety standards, a current limiting circuit is often added to the power supply circuit to prevent the fuse from blowing. There many current limiting topologies that may be employed on either a high potential or low potential side of a power supply. Most current limiting topologies include a resistor to convert current to voltage in order to provide feedback that is proportional to current. A comparison to a reference voltage is performed. The impedance of a series element is adjusted in response to the comparison. A problem with adding a current limiting circuit is that the conversion of current to voltage adds to the total output resistance and causes additional voltage loss beyond the voltage loss required to meet intrinsic safety standards.
The above and other problems are solved and an advance in the art is made by power supply having an integrated current source feedback and current limiting element in accordance with this invention. One advantage of an integrated current source feedback and current limiting element is the voltage loss may be minimized to the voltage that must be limited for intrinsic safety standards. A second advantage is that the number of components of a power supply circuit are reduced which lowers the cost of producing a power supply.
In accordance with the present invention, the function of a current conversion resistor in a power limiting circuit is combined with the function of barrier output resistance. This allows output resistance to be no more that the resistance required to prevent ignition of a hazardous material. The combination of function is provided by moving parts of the current limiting circuit to a point after the barrier resistance. In particular, a variable impedance device is moved to a point after the barrier resistance. One example of a variable impedance device is a MOFSET transistor.
When a MOFSET transistor is moved, there are two new paths to the output terminals. A first is a an op-amp control output and a feedback from a feedback from the barrier resistance. An input to an operational amplifier and the gate for the MOFSET transistor are of high impedance and relatively large value resistors compared to a barrier resistor placed in each of these paths. The total barrier resistance in a combination of resistor placed in a path with inputs into the MOFSET gate and operational amp input. This limits the power supply to an output resistance which is negligibly lower than the barrier resistance alone.
An aspect of the invention comprises an intrinsically safe circuit configured for supplying power to a load connected to a first output terminal and a second output terminal;
said circuit comprising;
a power supply source;
a voltage limiter connected in parallel with said power supply source to limit the maximum voltage generated by said power supply source;
a connection between a first side of said voltage limiter and said first terminal;
a current limiter comprising;
a reference voltage;
a variable impedance;
a barrier resistor;
a second side of said voltage limiter is connected in series with said barrier resistor and said variable impedance to said second output terminal;
said current limiter limits a current delivered to said load via said terminals to a maximum current level in response to a comparison of said reference voltage with a voltage across said barrier resistor representative of the current delivered to said load.
Preferably the intrinsically safe circuit further comprises a fuse connected between a positive voltage side of the power supply source and the voltage limiting circuit.
Preferably the voltage limiter comprises a diode that has a cathode connected to the positive side of the power supply source and further has an anode connected to the negative side of the power supply source.
Preferably the diode comprises a zener diode.
Preferably the current limiter and variable impedance comprises:
said variable impedance comprises a transistor for controlling the current level through the load, said transistor is connected to the second output terminal;
said operational amplifier controls the impedance of the transistor and has an output connected to a gate of the transistor, and further has a first input connected to a voltage representing said load current and further has a second input connected to said reference voltage;
a voltage divider including said barrier resistor for generating a voltage level that is representative of said load current, said voltage divider is connected to the first input of the operational amplifier, and to the negative side of the power supply source.
Preferably the current limiter further comprises a resistor connected between the output of the operational amplifier and the gate of the transistor.
Preferably the current limiting circuit has a transistor comprising Metal Oxide Semiconductor Field Effect Transistor (MOSFET).
Preferably the current limiter has a voltage divider comprising:
a first resistor connected between the first input of the operational amplifier and drain of the transistor; and
a second resistor having one end connected to said drain and a second end connected to the negative side of the power supply source.
Preferably the intrinsically safe circuit is characterized in that the load comprises electronics of the Coriolis flowmeter having a signal conditioner circuit for generating a drive signal and for receiving pick-off signals, said signal conditioner circuit is connected to the first and second output terminals of the intrinsically safe circuit.
Preferably the signal conditioner circuit comprises:
a drive circuit for generating said drive signal, said drive circuit is connected to the first and second output terminals of the intrinsically safe circuit; and
a conditioning circuit for receiving the pick-off signals, said conditioning circuit has an input connected to an output of the drive circuit.
Another aspect comprises a method of operating an intrinsically safe circuit for supplying power to a load;
said method comprises the steps of;
connecting a voltage limiter connected in parallel with a power supply source to limit the maximum voltage generated by said power supply source;
extending a connection between a first side of said voltage limiter and a first terminal;
connecting a second side of said voltage limiter in series with a barrier resistor and a variable impedance to a second output terminal;
with a barrier resistor and said variable impedance to said second output terminal;
limiting a current delivered to said load via said terminals to a maximum current level in response to a comparison of a reference voltage with a voltage across said barrier resistor representative of the current delivered to said load.
Preferably the step of limiting the voltage level includes operating a diode to limit said voltage.
Preferably the step of operating a diode includes the step of operating a zener diode.
Preferably the method further comprises the steps of:
applying intrinsically safe power from said terminal to a drive circuit of a Coriolis flowmeter; and
generating a drive signal with the drive circuit in response to the reception of said power.
Preferably the method further comprises the step of applying pick-off signals to a conditioning circuit of said Coriolis flowmeter in response to the reception of said power.
Preferably the method is characterized in that the step of limiting the current comprises the steps of:
operating a voltage divider to generate a voltage level that is representative of the load current;
operating the operational amplifier to compare a reference voltage with said voltage level representing the load current; and
extending a signal from an output of the operational amplifier to a control input of a transistor for limiting a current to a maximum level through the load.
Preferably the method is characterized in that the step of operating the transistor includes the step of operating a MOSFET.